1. Field of the Invention
The invention relates to an optical sensor for determining an analyte, specifically oxygen, with a sensor matrix that is formed substantially of a fluoropolymer and contains a luminescent indicator. In addition, the invention also relates to a method of manufacturing the sensor, a method of determining an analyte in a measuring medium, as well as an application of the optical sensor.
2. Prior Art Related to the Invention
Optical sensors are used in the most diverse fields of application to detect the presence of certain analytes in a measuring medium and in particular to measure their concentration. Examples of such analytes are oxygen, carbon dioxide, sulfur dioxide, as well as other chemical substances that are contained in a liquid or gaseous measuring medium, e.g., in an aqueous solution, in air, and other media.
Optical sensors within the same general field as the present invention are described in WO-93/18391 and U.S. Pat. No. 5,152,287, and also in a dissertation, xe2x80x9cOptical Sensors for Trace Oxygen Analysisxe2x80x9d, by Martina Trinkel, Department of Natural Science, Karl Franzens University, Graz, July 1998 (which will subsequently be referred to as the xe2x80x9cTrinkel dissertationxe2x80x9d). The optical sensors described in these references have a sensor matrix that is formed essentially of a fluoropolymer and contains a luminescent indicator. The luminescent indicator has the ability to be excited by light and to return some of the absorbed light energy as luminescent radiation, wherein one or more measurable properties, including the intensity or the decay period of the luminescence, are variable in response to the analyte that is to be determined. Because of the permeability of the sensor matrix for the analyte, a high analyte concentration in the measuring medium will cause a correspondingly high analyte concentration in the sensor matrix and thus manifest itself by a strong change in the luminescent properties, which can for example be expressed through the known Stern-Volmer equation. In principle, a measurement of the luminescent properties can be used to draw conclusions regarding the concentration of the analyte in the measuring medium.
The main requirements for a sensor matrix include a high degree of transparency for the light used to excite the luminescent indicator as well as for the radiation emitted by the luminescent indicator, a sufficient degree of permeability for the analyte, as well as mechanical and chemical durability.
The optical sensors described in WO-93/18391, which are intended in particular for the detection of oxygen or carbon dioxide in a fluid measuring medium, have a sensor matrix that is formed of a hardened, partially fluorinated polyurethane and contains a luminescent indicator of a known kind. In U.S. Pat. No. 5,152,287, a sensor matrix is described that is formed of a partially fluorinated polyacrylate and likewise contains a luminescent indicator of a known kind. In the manufacturing process of these known sensor matrices, the luminescent indicator used in each case, preferably a transition metal complex, is mixed with the basic materials required for producing the fluoropolymer, normally a precursor substance and a hardener substance. Subsequently, the fluoropolymer with the locked-in luminescent indicator is formed by thermal or light-induced cross-linking.
The known sensors described above suffer from the drawback that the fluoropolymer used in them as a matrix material does not have the chemical and/or mechanical durability required for certain applications. However, the use of a chemically and/or mechanically more durable fluoropolymer such as poly-tetrafluoro-ethylene or one of its known derivatives is made difficult, if not impossible, by the fact that the known luminescent indicators cannot be incorporated at all in the more durable fluoropolymer, or only with considerable difficulty. In particular, the known luminescent indicators are difficult to dissolve or totally insoluble in highly fluorinated or even perfluorinated polymers, so that a homogeneous distribution of the luminescent indicator molecules in the sensor matrix is difficult or even impossible to achieve. As a particular disadvantage, an aggregation of luminescent indicator molecules which takes place in a time span of days to weeks will cause a noticeable change of the optical sensor properties. In addition, one often observes a highly undesirable escape of the luminescent indicator from the sensor matrix. Prototypes of optical sensors for analyzing gases that are present in trace amounts are described in the Trinkel dissertation, where the sensor matrix consists of a copolymer formed of tetrafluoro-ethylene and 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxol. Fluoropolymers of this kind are known and available, e.g., under the trade name of Teflon(copyright) AF, in particular Teflon(copyright) AF 1600 or Teflon(copyright) AF 2400. A variety of known complexes of tin, palladium, and platinum-porphyrin, among others, were used as luminescent indicators in Trinkel""s work. As described in detail in the Trinkel dissertation, (see in particular chapter 4.4.6., xe2x80x9cStorage Stabilityxe2x80x9d) the sensor prototypes under investigation showed an undesirable escape of the luminescent indicator from the sensor matrix. Consequently, the investigated optical sensors, because of their inadequate durability, had to be rejected as being not suitable for process-monitoring (see last sentence of chapter 4.5., xe2x80x9cConclusionxe2x80x9d).
The methods that have until now been used in manufacturing the types of sensors described above are based on introducing the luminescent indicator already before forming the fluoropolymer. This has the disadvantage that during the polymerization process, the aforementioned non-homogeneous distribution of the luminescent indicator can occur while, on the other hand, the luminescent indicator may even be destroyed by the highly reactive radicals that participate in the polymerization. Furthermore, the known processes are expensive, they involve handling of toxic substances, and their reproducibility is poor. The Trinkel dissertation describes how attempts were made to circumvent these problems by using Teflon(copyright) AF, which is soluble in highly fluorinated and perfluorinated solvents, as a matrix material. As mentioned above, the attempt was unsuccessful because the resulting sensor matrices did not have the desired properties and in particular, because they showed an aggregation of the luminescent indicator and/or an escape of the luminescent indicator material from the sensor matrix.
The object of the present invention is to provide an improved optical sensor, as well as a method of manufacturing the sensor, that are free of the aforementioned drawbacks.
Further objects of the invention are to provide a method of determining an analyte in a measuring medium and to describe a use of the optical sensor according to the invention.
An improved optical sensor for determining an analyte, particularly for determining oxygen, with a sensor matrix that is formed substantially of a fluoropolymer and contains a luminescent indicator, is distinguished by the fact that the luminescent indicator contains a metal complex of ruthenium, rhenium, rhodium or iridium, and at least one at least partially fluorinated ligand.
In a method of manufacturing the optical sensor according to the invention, a quantity of the luminescent indicator and a quantity of the fluoropolymer are dissolved in a solvent, and the sensor matrix is formed subsequently by evaporating the solvent.
Further according to the invention, a method of determining an analyte in a measuring medium is based on the concept of bringing the inventive sensor into contact with the measuring medium and by detecting or measuring a change of an optical property of the luminescent indicator that is caused by an interaction with the analyte.
Lastly, the scope of the invention also covers the use of the inventive sensor for detecting or measuring oxygen in a liquid or gaseous medium.
With a luminescent indicator which, in accordance with the invention, contains a metal complex of ruthenium, rhenium, rhodium or iridium and at least one partially fluorinated ligand, the luminescent indicator can be introduced significantly better into fluoropolymers of all kinds. Thus, the selection of a particular fluoropolymer can be adapted to the range of applications for which the optical sensor is intended. In particular, the invention makes it possible to use highly fluorinated or even perfluorinated polymers. The metal complexes proposed by the invention with at least partially fluorinated ligands are distinguished by good solubility in fluoropolymers. Thus, they ensure a thermodynamically stable, homogeneous distribution of the luminescent indicator in the sensor matrix. As a result, the optical sensor according to the invention has a high degree of constancy and durability.
In the inventive method of manufacturing the sensor, a quantity of the luminescent indicator and a (different) quantity of the fluoropolymer are dissolved in a solvent, and the sensor matrix is subsequently formed by evaporating the solvent. In contrast to the known methods, the manufacturing process according to the invention uses a fluoropolymer procured in pre-manufactured form from a supplier. Because no polymerization processes have to be performed in the manufacture of the sensor, the inventive manufacturing method is simple to carry out, has good reproducibility, and in addition does not require the handling of toxic substances. In addition, the method according to the invention avoids the problem of the luminescent indicator being broken down by the radicals that participate in the polymerization processes and by other highly reactive species.
According to a preferred embodiment of the invention, the sensor matrix is formed of fluoropolymer which, in essence, is room-temperature soluble only in highly fluorinated or perfluorinated solvents. Preferably, the sensor matrix is formed of a derivative of poly-tetrafluoro-ethylene. Because the fluoropolymer is soluble exclusively in those solvents, there is assurance that the sensor matrix will not be dissolved or swell up on contact with another solvent that may be present in particular in the measuring medium. Particularly preferred is an embodiment of the invention where the fluoropolymer is a copolymer formed of tetrafluoro-ethylene and 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxol. Fluoropolymers of this kind are known, e.g., under the trade name of Teflon(copyright) AF, in particular Teflon(copyright) AF 1600 or Teflon(copyright) AF 2400, and are distinguished by a high degree of solubility in selected highly fluorinated or perfluorinated solvents. The high transparency of these fluoropolymers is a favorable factor for the optical excitation of the luminescent indicator and for the detection of the luminescent radiation. In addition, the high degree of permeability to gases assures a rapid response to changes in the concentration of the analyte in the measuring medium.
The scope of the invention also covers advantageous embodiments of the metal complex. Primarily two-pronged chelate ligands are proposed as ligands for the metal complex, with particular preference for the group of polypyridines, particularly the group of substituted 1,10-phenanthrolin derivatives and substituted bipyridines containing at least one at least partially fluorinated substituent. In particular, a 4,7-dipentafluoro-benzyloxy-1,10-phenanthrolin may be used as a ligand. An alternative choice for a ligand can consist of a 5-pentafluoro-benzamide-1,10-phenanthrolin; a 4,7-di-(3,3,4,4,5,5,5-heptafluoro-pentyl)-1,10-phenanthrolin; an N,Nxe2x80x2-di-(2,2,3,3,4,4,4-heptafluorobutyl)-1,10-phenanthrolin-4,7-diamide; or a 4,7-di-2,2,3,3,4,4,4-heptafluoro-butyl-amino-1,10-phenanthrolin. Another possible alternative for a ligand is a 4,4xe2x80x2-di-(3,3,4,4,5,5,5-heptafluoropentyl)-2,2xe2x80x2-bipyridine or an N,Nxe2x80x2-di-(2,2,3,3,4,4,4-heptafluorobutyl)-4,4xe2x80x2-bipyridine-2,2xe2x80x2-diamide. The metal complex can contain either a plurality of identical ligands or a plurality of different ligands. The preferred choice of a metal complex is a ruthenium(II)-tris-(4,7-dipentafluoro-benzyloxy-1,10-phenanthrolin; or a ruthenium(II)-bis-(4,7-diphenylphenanthrolin)-(4,7-dipentafluoro-benzyloxy-1,10-phenanthrolin); or a ruthenium(II)-bis-(4,7-dipentafluoro-benzyloxy-1,10-phenanthrolin)-(4,7-diphenyl-phenanthrolin).
In one embodiment of the invention, the luminescent indicator contains an at least partially fluorinated counter ion, which results in improved solubility of the luminescent indicator in the fluoropolymer that forms the sensor matrix. Particularly advantageous is an embodiment in which the counter ion is suitable for causing a steric hindrance. Examples of anions with this property are tetrakis-(4-chlorophenyl)-borate; tetrakis-[3,5-bis-(trifluormethyl)-phenyl]-borate; and tetrakis-[3,5-bis-(1,1,1,3,3,3-hexafluor-2-methoxy-2-propyl)-phenyl]-borate, which because of their bulk present a further obstacle to an undesirable aggregation of the luminescent indicator.
In a further advantageous embodiment, the sensor matrix is configured in the shape of a foil. This makes for a short entry path of the analyte into the sensor matrix, so that the optical sensor will have a rapid response.
In another embodiment, the sensor matrix is provided with a protective layer consisting substantially of the same fluoropolymer as the sensor matrix but containing essentially no luminescent indicator material. This serves to avoid a direct contact between peripheral portions of the sensor matrix and the measuring medium, so as to prevent mechanical damage to the sensor matrix from abrasive particles of the measuring medium and/or a reaction of the luminescent indicator with substances contained in the measuring medium.
A preferred version of the manufacturing method for the optical sensor involves the use of a highly fluorinated or perfluorinated solvent, e.g., octafluorotoluol, which has strong solvent capabilities for the fluoropolymers used in forming the sensor matrix as well as for the metal complexes with fluorinated ligands that are used as luminescent indicators.